Switchable tappets, which have spring means, as well as a spring support part, that is fixed axially at least in the force direction of the spring means to an inner part of the tappet with the help of a retaining ring forming a positive fit, are known in the state of the art. These tappets are used for turning off or switching the transmission of one or more cam strokes to a tappet push-rod of a valve drive of an internal combustion engine. For this purpose, such tappets have an inner part that can telescope relative to an outer part. The inner part can be locked to the outer part through a locking mechanism. To maintain contact between cam and tappet parts, whose movement in the unlocked state is not transmitted to the gas-exchange valve, a spring means is mounted between the inner part and the outer part.
Thus, in U.S. Pat. No. 6,802,288 B2, which is considered as class-defining, a switchable roller tappet is provided, as used more and more in large-volume V-engines with bottom camshafts and tappet push-rods as a valve drive-side basis for turning off cylinders as a measure for reducing the fuel consumption of the internal combustion engine. During decoupled outer part functioning, the tappet push-rod connected to the inner part remains stationary, wherein the mass action starting from the movement of the outer part is received by a so-called lost-motion spring. This spring is mounted between the outer part and a spring support part, which is fixed axially to the inner part by a retaining ring. Especially for switchable roller tappets according to the noted document, very high demands in terms of functional reliability, installation space, and assembly ease are placed on the axial fixation of the spring support part to the inner part. Thus, initially according to the relatively high mass of the outer part of the roller tappet, a lost-motion spring with high work capacity is to be used with simultaneously high spring forces, which the retaining ring must transmit to the inner part with resistance to wear and defects over the service life of the internal combustion engine. However, due to the very limited installation space available for the switchable roller tappet, which may typically be built no larger than a conventional non-switchable roller tappet, the axial fixation and thus the retaining ring and its surroundings are to be dimensioned as small as possible. In addition, the axial fixation is to be embodied so that it allows process-sure assembly.
With respect to these conditions and due to lack of detailed information, someone skilled in the art would understand from the noted document that the shown retaining ring involves a snap ring, which is lowered completely into an annular groove of the spring support part during assembly of the spring support part, in order to then snap into a corresponding annular groove of the inner part and to interact with this part in a positive fit. However, the assembly-related requirement of complete lowering of the retaining ring into the annular groove requires, on one hand, a sufficiently flexible retaining ring, thus with small cross-sectional dimensions, and, on the other hand, a sufficient radial play of the retaining ring in the annular groove of the spring support part. Both lead to an accumulation of high material stresses, because the small cross-sectional retaining ring as a rule snaps eccentrically into the annular groove of the inner part and consequently only small sections of its periphery are available for force transmission. In this respect, this type of axial fixation of the spring support part in connection with a switchable roller tappet is viewed as disadvantageous since the resulting large and pulsing forces cannot be transmitted permanently with sufficient reliability.